This invention relates generally to optical polarization splitters, and in particular to polarization splitters using birefringent elements.
The rapid acceptance of optical fiber has led to a variety of actual and proposed optical communications systems. Some of these include CATV, fiber to the home (FTTH), wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) systems as well as coherent communications systems. These technologies require versatile optical devices to perform various operations including isolating and routing of light beams. Many of the requisite devices, i.e., optical isolators and circulators, rely on polarization splitting to perform their functions. Specifically, many of these devices use a birefringent element, such as a birefringent crystal, to separate two orthogonal polarizations, e.g., the s-polarization and the p-polarization, contained in the beam. The separation occurs as the light beam propagates through the birefringent element.
The principles of birefringence or double refraction are well-known. Inside the birefringent material one of the orthogonal polarizations travels as an ordinary wave (o-wave) and the other travels as an extraordinary wave (e-wave). The o-wave experiences a different index of refraction than the e-wave; hence the two are refracted or bent by a different angle while propagating through the birefringent material. As a result, the e-wave is spatially separated from the o-wave and the separation or walk-off distance grows in a generally linear fashion with the distance traveled in the birefringent material.
After propagating through a length of birefringent medium the two orthogonal polarizations are thus walked-off by a sufficiently large distance that they can be processed separately by subsequent optical components. For example, typical optical circulators and isolators use reciprocal and non-reciprocal polarization rotating elements to perform different operations on the two walked off polarizations. Details about such devices can be found in numerous prior art publications, some of which include: Yohji Fuji, xe2x80x9cPolarization Independent Optical Circulator having High Isolation over a Wide Wavelength Rangexe2x80x9d, IEEE Photonics Technology Letters, Vol. 4, No. 2, February 1992; U.S. Pat. No. 5,471,340 to Cheng et al.; U.S. Pat. No. 5,319,483 to Krasinski; U.S. Pat. No. 5,212,586 to Van Delden; U.S. Pat. No. 4,464,022 to Emkey; U.S. Pat. No. 4,650,289 to Kuwahara; U.S. Pat. No. 5,204,771 to Koga; U.S. Pat. No. 5,689,593 to Pan, U.S. Pat. No. 5,930,039 to Li.
While expense and device dimensions are important in nearly all applications, they are particularly critical in high volume, low-cost projects such as FTTH. The polarization splitter (which functions as a polarization combiner for returning polarizations) used in the present devices utilizes a long birefringent crystal to achieve sufficient walk-off for further processing. Hence, the size and cost of the devices built with present polarization splitters present an obstacle to further advances in optical devices.
In view of the above, what is required is a polarization splitter or combiner which achieves large walk-off over short distances. This would permit further miniaturization of optical devices which require polarization separation. Additionally, a smaller polarization splitter would use shorter blocks of birefringent material and thus be more cost effective.
Accordingly, it is a primary object of the present invention to provide an apparatus for spatial polarization separation, e.g., a polarization splitter and combiner which utilizes a short birefringent element. The splitter of the invention advantageously achieves a significant walk-off distance between two polarizations over a short length.
It is a further object of the invention to provide a polarization splitter which is low-cost and has small dimensions. The splitter should be easy to deploy in optical devices which require polarization splitting and/or combining.
The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below.
These objects and advantages are secured by a polarization splitter for processing a beam containing a first polarization and a second polarization to spatially separate these polarizations. Typically, those two polarizations will be orthogonal, such as the s- and p-polarizations. The splitter has a first optical element, e.g., an optical fiber, for delivering the beam and a birefringent element for receiving the beam and walking off the first polarization from the second polarization by an initial walk-off distance. A first lensing element such as a graded index lens (GRIN) with an optical axis is oriented to admit the first polarization at a first input point and output the first polarization at a first output angle to the optical axis. Likewise, the first lensing element admits the second polarization at a second input point and outputs it at a second output angle to the optical axis. The first and second input points are separated by the initial walk-off distance.
To enjoy the full benefits of the splitter of the invention, it preferably has a second lensing element, which can also be a GRIN lens, positioned after the first lensing element. The second lensing element is set up to receive the first polarization at a first input angle equal to the first output angle from the first lensing element. The second lensing element also receives the second polarization at a second input angle equal to the second output angle, and outputs both polarizations at a first and second output point respectively. In a symmetrical arrangement, the first and second output points can be separated by the initial walk-off distance. A second and third optical elements, e.g., second and third optical fibers, can be located at the first and second output points to receive the separated first and second polarizations respectively. When using fibers, it is advantageous to use sleeves with appropriate bores for holding them.
The splitter can have additional optics located between the first and second lensing elements. These optics can include devices which perform polarization rotation functions, filtering functions or indeed any desired functions of the first and second polarizations. However, it is important to ensure that the first and second output angles from the first lensing element are equal to the first and second input angles at the second lensing element.
Additionally, the walk-off or spatial separation between the polarizations should be controlled. A control device, such as a second birefringent element can serve this function.
In one embodiment, the first lensing element of the splitter can be positioned such that the first input point is on the optical axis. In this situation, the first output angle is zero.
In order to limit losses the elements of the splitter can be positioned at slant angles. For example, the birefringent element can be mounted at a slant angle xcex4. Likewise, the lensing elements can have input facets inclined at slant angle xcex4.
The polarization splitter can be used as a polarization combiner in accordance with the invention to combine the first and second polarizations emitted from two optical elements, e.g., the second and third optical fibers. The first lensing element performs its function by admitting the first polarization at a first input angle to its optical axis and outputting it at a first output point. The second polarization is admitted at a second input angle to the optical axis and output at a second output point. The output points are separated by the initial walk-off distance, such that the birefringent element combines the polarizations to form the beam which is supplied to the first optical element or first optical fiber.
The combiner can have the same additional elements as the splitter as well as additional optics including additional birefringent elements positioned between the first and second lensing elements.
The splitter and combiner of the invention can be used in more complex optical devices which employ the splitter and combiner for separating polarizations before performing other functions.
The particulars of the invention and its various embodiments are described in detail in the detailed description section with reference to the attached drawing figures.